This invention relates to a system for shift control in automatic transmission, and particularly to improvements in a system for shift control in automatic transmission, wherein the durability of frictionally engaging devices can be secured by a so-called failsafe function.
Automatic transmissions comprising gear transmission mechanisms, a plurality of frictionally engaging devices, and hydraulic pressure control devices operated to selectively switch the engagements of the frictionally engaging devices, so that any one of plurality of gear stages can be achieved in accordance with a preset shift map, are well known in the transmission art.
Furthermore, in an automatic transmission for a vehicle, of the type described, various systems are known for integrally controlling an automatic transmission and an engine, wherein engine torque is changed during shifting to obtain satisfactory shift characteristics and durability of the frictionally engaging devices (For example, Japanese Patent Laid-Open No. 69738/1980). More specifically, the system for integrally controlling the automatic transmission and the engine of the type described changes the amount of torque transmitted from the engine during shifting and controls the amount of energy absorbed by various members in the automatic transmission or by the frictionally engaging devices so as to complete a shifting within a short period of time under a low shift shock, whereby a satisfactory shift feeling is given to a driver and durability of the frictionally engaging devices is improved.
However, when the control for changing engine torque cannot be carried out due to trouble in a sensor system or a request from the engine side, durability of the frictionally engaging devices is reduced due to an increase in the amount of energy absorbed by the frictionally engaging devices in the automatic transmission. Moreover, shifting time duration is prolonged thereby worsening the shift feeling. This is caused by the shift tuning data (oil pressure and the like) in the automatic transmission set in expectation of a decrease in the engine torque by a predetermined value during the shifting.
Furthermore, no matter whether or not the engine torque change control during shifting as described above may be carried out, when the working oil pressure is decreased due to the oil leakage for example, the shift time duration is also prolonged to affect less durability of the frictionally engaging devices, and the shifting is not completed in a shockless region of the accumulator, whereby high shift shocks occur.